Method of measuring light.



No. 888.802. .PATENTED MAY 26, 19 08;-

' w. J: HAMMER.

METHOD OF MEASURING LIGHT.

APPLICATION FILED OUT. 31.19.05.

lmm

HVVENTOR common forms .the estimates )vith the best substantiallwith-int J. nimnsn.

I Pi TliIN-T hli l. C E.

METHOD OF-MEASURING LIGHT.

No. 888,802. Specification of Application filed October 31, 1906.

1 all 101mm if may conccrrz;

Be it known that l, WiLLL-ui J. ll-iiipunma citizen of the UnitedStates, residing in New York city, in the State of New York, haveinvented certain new and useful lmprove ments in ...\lethods ofMeasuring which the following is a specification. My present inventionrelates to the art of measuring light, and particularly to an int,)rovement in that art by which lights may be compared withoutintroducing able physiological factor present when estimating ightintensities by the of photometers now in use.

to as comparison. of the lights to be ort-ance, inasmuch as not only dothe colors ditlier in the intensituof the effect produced upon the cyeof an iinlividual, but there is marked variation between differentpersons in this respect; so that bv some observers two lights of thesame color may be'coln: pared with considerable accuracv, but a changein the: color of the standard will renderthe' determination worthless. iimpossibility of maintaina standardlight which will not var Furthermore,the colors compared are of great in from time to timc with the pressureof the specil atmosphere and its with the place of test, and

flame (a amc lie-- mg u'suall employed-for this purpose), offacts theprecision of comparison toa greater or less degree. The common'standardof com a-ris'on ls 1 1e now well known Pentane amp; the

colorof this standard is toward the red end practice to have 'a pare anumber .of meandescent lamps wl he the selected ary standards, whichwhen burned. at-

give for a. considerable unvarymg light.

Voltage will Obviously,

Light, of;

the varieye, as in the 2 available scientific means of 5 7 ing thechanges in the current,

' any investigation 1t is best measuring devices Patented May 26, 1908.

this introduces the lamps will, in spite of all diil'er amongthemselves, and made with them will necesy thejpersonnl equal ions ol" 1he different observers. In addition these equations varyin differentways with dill'crent persons, being dependent as already pointed outupon physlo however, l'curacies, since possible care. compz'irisons-sarily dil'l'er b oglcal factors such as the l ocular perception of theindividual, varying with fatigue and with physical condition, as 5 iswell understood. All of these difficulties 5 the improved method now tobe set out; i which consists, briefly, in causing the light to aselected physical and measuring the variations so recision with whichthe meas- )e effect-ed may he made verv I aim to obviate by 1 urenicntcan great b takmg necessary precautions, mill 1s who] y lndependent ofthe ccuhurities of I different individuals, or -at .il-slmar be made soby proper care. i will be apparentfrom a consideration of the 1 presentstate of the art setout above,- I prefer that the quantity to e measuredshall be one the variations in which shall not be dependent upon anyphysiological function for I their accurate perception. Y l Severaldifferent ways of embod ing the ;invention are possible. The simp estand 1 best of all those that have occurred to me I 'anging, in anelectric circuit a selenium' cell approved construction through which Ipass a current, and

Man upon the cell of t test; it being well known that the specificelectric resistance of selenium varies wide, In

measuring instrument. I I pose an instrument which will show variationin only one electrical quantity, since in l for well known reasons toemployonlyone variable at atime;

ler some con may be used; f my invention I have waves and a y th g -w s;

directly but this, although far more accurate than 1 direct comparisons:oflight by the 'mor dilficult and andwith no compensating a vantage.

For reasons which i he source of currenti is included any desirableformof electrical prefer for this pur.

ditions other forms of more com lex to practice stilllurther inaem tocause eye, c

"40' -wellunderstood.

341,425 filed October 31st, 1906.

In the accompanying drawings Figure 1 is a diagram illustrating thelirs-t of the methods above indicated in my statement of invention, andFigs. 2, S and 4, are modified forms in which sound-wavesareemployed. vIn Fig. 1, A isthe battery or other source of current which is to bemeasured and varia-. tions in which are taken as indicative of lightintensities. The batter is-arranged in-cir'-' 'cuit'with a selenium cellB, which is preferably of that form in which the selenium,- afterbeingcoated upon a suitable conducting support, such as a coilqof nickelwire, is

sealed in a tube of preferably as riearlyperfeet a vacuum as can beobtained, the ends trolling means,' since obvious y such devices ma beemployed as the expert may-desire,- an customarily the current will beregulated by the usual resistances and switches in ways Preferablythecurrent through the cell should-be .of-as low aval'ue,

as can be conveniently maintained constant,v

for scveralreas'ons, one of Which-is that a lar e current evolves somuch heat that the eel is subjected to alternate expansions" and'contractions'which are detrimental; but the most important reason isthat for very small currents a proportionately. larger variation ofvoltage isjcaused vby the action of. the light upon the cell, renderingthe device more sensitive and accurate.

'Upon a suitable table I), is arranged the lamp E, to be tested. .At F,I 'have indi rated a, screenhaving a ho le f which may be closed :bya-slide F; in practice it is prefer able to place the selen um cell inalightv ftightlbox M -,(shown partly broken away) having freeventilatiomso that the indications may notgbetafiected by exteriorlightg;

and 'inasmuch as the efi'eet to be observed is not detectable by etjamina'tionofthe cell, it is onlnecessary. to "openthe box at long-inftervas. v

he method of operating thej"ail'r,ari 'gemeiit guard against change. r

show a second meansof 'cat'ed.

.tion adjacent to the apparatus and lighted, being screened completelyfrom the cell; a

small current, referably a small fraction of an ampere, is t ion causedto How in the cir-' cuit including the cell, and after it has attaineda' steady value, the fall of potential around the cell is measured bythe volt-meter. After this the light of the standard lamp is allowed tofell upon the cell, and the change in the.

resistance of the circuit (as indicated by the change in the drop)caused by the action of the light upon the selenium is measured andrecorded; this then becomes'a constant of the articular cell employed.After this, the lig t which is to be compared with the standard may haveitsspecificeffect upon the cell determined in substantially the sameway. Obviously the indicationsofthe cellfor thefirst specimensconstructed r'nust be calibrated by comparison with the results obtainedby 'photometers, since the meas urements of light now in use are purelarbitrary and have no relation to any efinite hysic'a quantity; but thisrelation having cated may be indefinitely perpetuated by periodiccomparisons of different cells to In Figs. 2 and 3, 7 practicing themethod'whieh I have indilhis. consists of a siren-disk G of well knownconstruction in which the rows of holes 9 gbear to each other certaindefinite relations, such asthe notes of'the scale. 01p; posit e to, androtating with this disk (t e perforated disk being shown detached forclearness of illustration) is a disk H bearing upon-its surfaceconductors coated with selenium {one for each of, the rows-pf holes g.

driven at constant speed; Inthe circuit with the collector rings andsource of current supply isa coil l, disposed so as to afl'ectatumng-Ifork K provided with a ointer 7c, the" amplit-de of 'v1bration ofwhic is recorded.

upon the revolving cylinder L, in the manner 0 a-fchronographp The rowsof holes in the scale, and may eachbe provided'vvith a simisiremdisk mayconveniently be the rela tion to the notes of the octave of a tempered31.20

lar tuning-fork, which at the normal rate of "rotation will respond toi-ts-ap ropriate tone;

'these being. all substantially he the fork K v and arranged in asimilar way, are not illustrated Such ,an' organization will enable t'hesame time, when desired] The apparatus thus, described beingniaintained' as nearly constant as possible,

125 the observer to'co'nduct a number of tests at ear of the observer;an operator with a good musical car may reach a very close a proximationto a correct comparison by t e relative-intensities of the soundsproduced by the two lights; this is, however, open (though in a lessdegree) to the objections pointed-out with respect to visualphotomet'ers.

InFig. 4,-I indicate a construction which is cheaper and etsubstantially efiective. In this figure theeells B are arranged upon astationary supportand the siren-disk alone rotates, cutting'oif'andadmitting the light by its rotation.

Having now indicated the general outline of the apparatus which I preferto use and which by anyone familiar with physical manipulations maybealmost indefinitely varied in detail Without departing from. myinvention, I will now proceed to describe some of the precautions whichI find it desirable to use in order to'effect the measurements towhich Ihave referred with the greatest facility and precision. Obviously theinvention will be employed although the arrangements whichI am now aboutto indicate should be omitted; but they add so greatly to the precisionof the result that in most cases it will be preferable to employ them. r3

I have already 'mentioned that the selenium cell should be inclosed in avacuum; the

reason for' this is that the selenium is hy groscopie and when dampthere is in such a cell a certain electrolytic action between theselenium and its metallic support which gives rise to an electromotiveforce in the cell itself, making it to a certain extent a primarybattery which may polarize, so that its E. M. F. varies between certainlimits, thus Whatever amount of electric effect may be present. Inaddition the slight heating of the cell causes it to give out gases,changing its internal resistance and'again affecting its E. M. F. Thesedeleterious effects are wholly avoided by inclosing the cell in a highlyexhausted receiver; and if in the course of this process a current bepassed through the cell suflicient to heat it, at the same, timecontinuing the action of the pump, it remains constant for a long periodof tune'iifI have also found it of advantage to makethe inclosingtubefor the selenium cell 6f fused quartz, which has the property oftransmitting some rays which are verydargely excluded by glass, andwhich have a considerable action upon the selenium; this is, so farentirely new with me.

I do not in this a plication, claim tlfiSCOllstruction of the seleniumcell itself, which I have found best adapted to my improved I know,

method, since it is described and claimed in adapted to the purpose;Brass and copper wire may also be used; but they are objec- .tionablebecause a selenite of copper is formed between which and the copper asmall E. M. F. may be generated, impairing the accuracy and durabilityof the cell.

Another precaution which should be em ployed is to standardize thesurface of emis-' sion of light so that the same extent of surface ofthe standard and of the light-source to be measured will be exposed tothe selenium cell. One of the best methods of accomplishing' this is toscreen the selenium from the light and pass the-portion of light to bemeasured through a tube the inner surface of which is blackened, of adefinite area of cross-section, such as one square millimeter, or insome cases one ;'square centimeter, though iii general the latter unitwill be found somewhat large. Obviously, however, whatever unit ofsurface be chosen should be the same for all of the lights which 'are'tobe com ared. Under some circumstances, as w ere the average horizontalcandle-power of a rotating lamp is to be taken, the larger surface ofexposure may be found most'convenient.

The circuit in which the cell is interpolated should be of very highresistance, not only because this cuts down the current to a very smallvalue, but because it tends to steady it and make it more responsive tothe effect to be measured, while less liable to disturbance from othercauses. I have used for this purpose resistance as high as 10,000 ohms;and asthe resistances of the selenium cell itself is very' high, thetotal resistance in the circuit. may be very large indeed. YVith acircuit of these characteristics, I have been by'special apparatusenabled to start and stop large electrical machinery by passing my handbetween the cell and the source of light. for somewhat similar reasonsthe volt-meter or galvanoineter employed should be one of largeresistance.

There are other methods of employing measurable physical quantities forthe pur poses set out, and all of these which employ Also ' n'amtsul'inglight, which it to be nmmureil phya-li ml qu lmtity,

vr: illtion.

I'JMBELhB'LlTll'IgJ; light, which lruusiiilis-i in mm 'Vttl.lttlfi)l1 ii 2111 amcertaihllhla ply. ntity lTll' otltlfiflmll'by a stmillz'll'lllight, mull thlm llltiftizlllll'ill 5 1' thl'i shini- Ila-r variationprmluqml .hy the .li ht to he GOlLllPfitli'flLl. with the stm'lllzlrrl.

3. This new art of imstsillriilg l Mir; cumrellt I (:(Yl'lsists incausing the light to vary elem mvii'ig in u, (Zilfflllill, and meas-111M110 the extent Oil that variation.

4;. The new art of mhiisuring light, which GUHSlHhS in causing the light01, :1, stmnlaml lamp to uni the cum n't flow inimi .elvutrim c-itmlit,:"Ll'llll nu ing the extent 0'1 such Vatriathm, than musing at similarvariation by 1110mm ml" the light to he Lf/LH'HIMLI'GLl with the;

sliun lnrll, and measuring ita extent;

' .l H witnvq wl'iereof l. have hereunto set my 1: :1, me in 'thpresence of two Wi QI'IGSSGE'S.

I WILLIAM J. I'IAINHQIER. l l itnessesg I IRVING M1. UBRLEGHT, ll-MumBURKE.

